scrape method extracts external data and
stores it in a SqliteDataStore.scrapes is used to
determine which resources are "scrapeable" (as opposed to
to "refined" resources). For each of the resources,
query is called (with ignore.db=TRUE) and thus the
data extraction and transformation is performed. The
result of the query is stored in the datastore.
Usually the method needs not be overwritten by inherited
classes.